Octobo 1945
Wirelesm World
EXTRA-TERRESTRIAL RELAYS
Can Rocket Stations Give World-wide Radio Coverage?
LTHOUCH it ir posible, by
A
a ruitable choice of freq u e n u a and mutes. to pmvide telephony circuits betweui
any two points or regions of the
earth for a larga part of the time,
long-distance communication u
areatly hampered by the peculiari t i a of the ionmphere, and there
are even occasions when i t muy
be impouible. A troe broadcast
service. giving comtant held
strength a t al1 t i m a over the
whole globe would be invaluable.
not to say indisperisable. in a
world saiety.
Untatkfactory though the telephony and telegraph pceition h.
that of tdevision ir far worse,
since ionospheric baummsion
cannot be employed a t aii. The
K M C ~ area of a t e l d o n station.
even on a very good dte, u only
about a hundred mila acrou. To
cover a smaii country mch u
Grtat Britain would rcquirc a nettnrurnitten. connected
c
d lina, wavegnida or
rciay
O'.
iinks. A x e n t theoretical study' has &oan that mch
a system would requin repeaten
a t intervlL of ñfty d a or I m .
A systcm of tU kind could provide televkion covenge. a t a very
coiuidaable cost. over the whole
of a 4 country. I t wonld be
out of the quation to provide a
iarge continant with mch a service, aad only the mpín centre.
of popuktion c o d d be bsduded in
the netaork.
The p b l e m u cquaiiy serioiu
when an attcmpl b made to link
television servica in diñeruit
parh of the globc. A reiay chain
several thoiuand m ü a long wonld
cost milliom. and tranxxeanic
servica would stül be impossible.
S i a r considerations apply to
the pmvision of wide-band frequency modulation and other servica. such as high-speed facúmile
which are by their nature nstricted to the ultra-high-frequencia.
Many may consider the solution
proposed in this dixussion too farfetched to be taken very seriously.
Such an attitude is unrcasonable.
as everything envisaged here is a
Z
By ARTHUR C. CLARKE
logial extension of developmentr
in the iast ten y-in
partic*
the perfation of the long-range
m k e t of which V2 au the pmtotype. Wbüe thia ariicle vu k g
writtcn. it w a announccd that the
Gennana w e n considering a similar pmjcct. which they believed
possible within Mty to a hundred
Y.Befom pmceeding further. it u
nu-ry
t o discuu brie%ycestain
fundamentai
of m k e t pmpulsion and m n a u t i u . "
A
m k e t which achieved a nifñuentiy great rprrd in Bight outside thc earh's atmosphere woul$
never retnrn. ibis " orbital
velocity u 8 km per wc. (5 miia
per S), and a rocket which
attained it would become an areificiai steiiite, Urrling the worid
for ever with no upuiditnre o?
p w e r - a wcond mwn. in fact.
kws
Fig.
1.
the atmcqhem aad left to b m d cast scientiñc information bpck to
the urrh. A littie iater. msnned
rockets wiii be able to rmke simiiar tiights with suñcient excm
power to break the orbit and mturn to euth.
T h e n are an inñnite nrimber o f .
posible stable orbitr. circniar and
eliiptical. in which a mdret w o d
rema& if tha iniorl conditiom
w e n corrat. I h e velocity of
S ian/.ec. a p p i h d y ta the
cl-t
posible orbit. one just outYde thC atmasphcre. rnd the
period of revolution wouid be
about 90 +uta.
Ilr the n d i w
of the orbit uicrrua the velccity
dccrrues. rince graviíy u diminUhing and l e s ccntrifiigrl force ir
needed to brlínce it. Fig. r &otU gnohicaiiy. The moon. of
counc. S a p u h c P l u casa rnd
wouid lie on the c u r v a of Fig. I
if they wen produced. The prop d Ge-e--tia
.
Vuiation of orbitai period uid reloaty with distan- fr0m
the e n h e of the euLh.
The Gennan bamatlantic rgcket
Aro would have reached m o n
than half thb velocity.
It wU be possibie in a few more
yean to build radio conhiied
rockets which can be s t e e d into
such orbib beyond the limib of
would have a period of about four
and a hall h o m .
1t wu be obxrved that one
orbit. with a radius of 42.h.
b a period of ucactly w h o m if i b
A body in mch an
plane coincided with that 0f
Wireless World
Ocbbcr 1945
~ B d w I earth's quator. would revolve
with the urth and woald thur be
stationary above the same spot
on the plrnet. I t w d d nmiin
ñxed in the sky of a whole hemis p h m and unlike di other
h u v e d y hodiu would neither rise
nor set A body in a d e r orbit
woald revolve more quiddy thui
the e a N i and $0 would Ne in the
wat. u indetd happau with the
inner moon of Man.
UPng materiai femed a p by
rockcts. it wyid be posib? to
cotutruct a *e-station
in
such an orbit. The sbtion c d d
be provided with living qautcn.
labontoria
rnd
everything
needed for tha comfort of i b c m .
wh0 W O U ~be~ r'die~edu i d Diovisioned by a regular rocket -servife. m pmject might & a d e r taken for p d y rimtific ravms
w it wouid contribute cno1l11011dy
to o m howledge of utmnomy.
phand metcorology. A good
d d of literature has a i m d y been
written on the subject.'
Although su& an undahking
may seem fantasac. it req*
Fii.-2..
~
b
menb muld be vcry small, u
d
i line of sight trmsmllsion
would be used. Thuc is the
further important point that
urap on the earth. once xt up.
could remain k e d indeñnitely.
Monover. a tnnsmiuion r+
ceived from any point on the
hembpherc could be b r o a d d to
the whole of the visible faca of
m&
vhpitc *tioar
muld-e
dobe.
the globe. and thru the rqnirammtr of d pornile
would be mct (Fi.
zZ.
I t may be v e d that we baw
u yet no dirat evidof rrdio
wava passing
th+ mrf-
Typic.1
-
-
complete caray. oi th
kcn re4
'
d
0
a
ceived fmm
in or a
l
~
theFJayer. Itseemrñllfyortriri
that frrqaancia h.
=y, 50
Mcls 0 xoo.000 Ahls d d be
rued witboat undae abmqtion in
the abmsphete or the ionosphae.
A siningle station wuid d y pn~.
vide coverr(la to hrlf the giobe.
rnd for a worid
thrre
woiild be nqoirsd. thoogh more
couid be rudiiy
Fig. 3
shom the rimplat a m a g a n a t .
Tha *krwld be &aaged
a p p r o x i m a t e l y quidkhntly
amund the d.
and the fdlovingiooeit<iderrppartoanitable:
3oE-E
.
150E-Chuu. rnd a
-.
go W-The h m c u
ThesbtiOnrinthschrinmcild
be linked by radio or opticzl
kimJ. and thur aay conceinble
bam or brovdcut s a v i a could
be provided. .
The technicrl problnns involved in the daign 9 III~~.?
t i o n r t r c ~ d y ~ .
bat only a few a n be gona into
here. B a t e a of p ~ r b o l i cre-'
%&bn
would be p v i d e d , of
a p r h u u depending on the +eq u a a a employed. Amunuig
the use of 3.ooo Mcls -va.
m i m n about a metre acros
would kam h o s t di the power
on to ths earth. iarger rt0ect o n could be used to Uuminate
si+ d
a or regiow foz the
mon reshicted $CMCCY,
with conli]rar
-
0
whole hcmuphcre.
for itr fdúiment rafrctr oniy
tviceuhrtuthomllradyin
the daign rtyla. .Since thc gnwitationrl stxula involved in the
rtrnctum are negiigible. only tha
very lightut xnahids wouid be
necanry ind tha sbtion cauid be
as iupe u requixed.
Let us now suppcae that su&
a station werc built in thll orbit.
It wuld be pmvided mth rueiving and transmitting qaipment
(the problem of power wüi be &
c u d later) and could act as a
repeater to relay transmiuions between rny two poinb on the
hemisphere benath. using any
frequency which wiU penetrate the
ionosphere. If dircctive anap
were uud. the power require-
necevidence by expIoring
for echoes fmm the mwn. In the
meuitime we have etuievidence
that fkquencia at the opticil end
of the qectcum pam throngh with
little absarption except at certain
frcquenaa at which raonance
&e&
acnr. Medium high frequenciu go thmagh the E layer
hvice to be iedected from the F
ofthscuthrndouterspace:di
cm my m t h carhbiy n that
the ahorter w a d @
are not
re%ectedback to thc Luth. Di&t
evidence of deld rtrrngth above
the u r t h ' s ' a t m o p h a s wuld be
obhined by Vz rccket tcchnique,
and it u to be hoped that wmeone
wiU do something &out this xx>n
as there m& be quite a surpltu
somcwhers 1 Alteniatively,
g t v e n mfñcicnt tnmmitting
power, we might obtain the
+
ZPd
October 1945
Wireless World
sequent economy of power. On
the higher frquencies i t is not
difficult to produce beanu l a s
than a degree in width. and. as
nientioned before. t h m would be
no physical limihtions on the
size of the mirron. (From the
space station. the d k of the arth
would be a little o v a 17 degrees
across). The wme minon could
be used for many different transmissiom if precautions were taken
to avoid c r w moduhtion.
I t ir clear h m the nature of
the system that the power needed
wiil be much l a r than that required for any other arrangement. since all the energy radiated can be uniformly d i i b u t e d
over the service asea. and none
is wasted. An approximate =timate of the power required for
the broadcast service h m a single
station can be made as foliows :The field *en@
in the equatorial plane of a h / z d i p o l ~in h e
space a t a d i i n c e of d metru
is
JT
e = 6.85 -vol/ metre, where
d
P ir the power ndiuted in watts.
Taking d as 42.000 km (effectively it would be l a ) , we h a y
P-37.6r'watts.
(#nOwin
rV / mebe.)
If we assume 8 t o be 50 microvolts/meize. which u the F.C.C.
standard for frequmcy modulation, P will he 94 k W . T h h h the
power required for a sinde dipole.
and not an a m y which would
concentrate al1 the power on the
earth. Such an a n a y would have
a gain over a simple dipole oí
about 80. The porver required for
the bmadcasr service would thus
l>e about 1.2 kW.
Ridiculously s t i d l though i t is,
t h k figure is probablv much too
genemus. Smail pa&bolas about
a f w t in diameta would be d
for receiving a t the a r t h end and
would give a very good signall
noise ratio. There would be very
little interference. partly because
of the frquency used and p d y
because the mirron would k
poinhng towa*
the sky which
could contain no other source of
signal. A 6eld seen@ of 10
mcrovoltr/metre might weli be
ample. and&.t
would rquire a
transmitter output of only 50
watts.
When it U remembered that
these figura relate to the broadc u t service. the efñciency of the
system will be r d i . The .pointto-point
beam
tranunurions
might need p o w m of only 10
watts or lo. T h a e figures, of
coune. would need correction for
ionospheric and atmospheric absorption. but that would be quite
small over most of the band. The
slight falling off in field strength
due to this cause towards ths
edge of the service u e a could be
readily correaed by a non-unifonn ndiator.
Ihc e5ciency of the system is
strikingly revealeti when we consider that the London Televuion
scrvice required about 3 kW
average power for M atea less
than iifty miles in radius.'
h suond fundamental problem
ir the provision of electrical
cnergy to tun the large number of
tnnsmitten required for the
different servica. In space beyond the atmosphere. a square
metre normai to the solar radiation intercepe 1.35 kW of energy.'
Solar eogina have alrcady been
devised for termtrial use and are
an economic pmposition in trooical countria.
They empLy
m h n to concentrate mnlight on
the ,boiler of a low-prusure steam
engine. Although thk anangement is not very efñcient it codd
be made much more so in space
wbere the operating componenb
are in a vacuum. the radktion is
intense and continuous. uid the
low-temperature end of rhe cyde
could be not íar h m absolute
zem. Thermollectric and photoelectric developments may make
i t posible to u t i h e the solar
energy more directly.
Though there u no limit to the
size of the mirron tbat could be
buüt. one fifty metru in radius
would intercept over ro.ooo kW
and a t least a quarter of thir
energy rhould be avaikble for we.
The m t i o n would be in continaoer d g h t except for some
weelu around the equinoxer. when
it would enter the e u t h ' s sbadow
for a few minuta every day.
Fig. .+s h o m thc state of a f h i n
during the edipw period. For
Wireless World
w o d d be avaiiable.
(3) The power requirements are
extremely small since the efaciency of " üiumination" wüi be
aimost IOO per ccnt. Morcover.
the cost of the power would be
very loa.
(4) Howwer great the initiai
upaue. it wonld o d y be a h c tion of t h a t requixed for the
ever. owing to i b ñnite .ceelention. tbe &et 10- vrlocity u r
m n i t of grrvitrtionai rrtudation.
If itr acceieration (usruned constant) u = metres/sec.'. then tha
neceuuy ntio R, h increased to
Chrpneia
t o consider the earth a?, ñxed and
the sun as moving round it. The
station w o d d graze the earth's
shadow a t A. on the last &y in
February. Erery &y. as i t d e
i b diurna1 revdution. i t would
c u t more dceply into the shadow.
undergoing i b pcriod of maxi-
O c b b a 1945
R.=
R-
a
+l
a
For an automatiuüy conkolled
rocket a would ba aboot 38 md so
?.he n u o t u y R wodd ba 37 to t.
Such ntico c a ~ o ba
t rrrliwd with
a Ungls &et
but can be athined
by
rtcprockeb"',
while vag
muda higher n t h (up to 1 . a ~to
x ) can bu achieved by tbe prinaple
of cellolu c o m c t i o n "*.
"
"
EpilogueAtomic Power
The advent of atDmic p a hr
a t one b o n d bmnght spce tnvei
M a centnry
It rsmu unUely that we d
i hava to wait aa
much aa twentp. vcuo befors
atomk-wwad Í&tr
are dee
ea-vi
-.
1I
r-
-
---
Fig. 4. Clac ndi.tion d
d i10 C U t O* f0r a rhort puiod uch da7 at
hepiiinrmz
duhiem
fór x hour
g &uta.
From thm onrudr the peziod of
eclipse wouid allortm. ind &ter
Aprii xxth (B)the statiw would
be in c01ltio.llousw.llight again
mtií tha suns thing happmed
six monthr iata a t the autrimn
eqpiaox. betran septemba 12th
rnd OItokr 14th. Tila tobl
penod of drrimas rorild be a b n t
two &ys par y a r . rnd u tha
10Pgat period of edips woold be
littie mors than ;in hour thcis
s h o d d be m d i f ñ d t y in rtoring
enongh poaa for an oniatcrrupted rrvice.
Condwion
B r i e e mmmPrised, thc adM n t a g a of the space station are
a?, folIow3 :(1) It u the only way in which
+me
world average can be
achieved for ali pamble typw of
se~ce.
(2) It permib annstrictcd w
of a baad at lea& ra>.oco Mc/s
of beam
mda. and with the
a n almost uniimited number of
*
nm-
a iantuticsii9 r m a ~hJ/- ntio
4 n l y d feá p a cent. The eqnation. devdooed in &e aoocndix süli
hold. but ;
wiii ba in;ñued by ,a
factor of rbont a thouund.
In view of theas f.ctr. i t a p p
hvdly worth ~ h i l em expnd-mnch
on the buüdin of l a g &
anea reiaw cimins. $ven tha 14
Rsi" Iwio.Rr,la
Appdii-~oc*.t D e s e
The d e d o ent of rockab 4-
SyYtalu."
C.
.w.
HÍprelL Bloc. I . R J . , Vol 33.
Msrch. 1&5.
"Rodrsb."
m y Ley. (VIkhig
poreStq,"orbia"
Presa. N.Y.)
rrid a v a "m d d t y ir nor
"
Daa
Problun,
des Behhmng de$
ozi~yr mttar
~ h folios
weitaalm¶.
Hq!nam Nooring &proa may be of intarut in t
e
. dung.
W
~
P
T h e r o c ~ h u f o ~ o i n a 5 l u l "Freqrimcy MduhW.'' A.
Hmd. (McCnw Hm.)
velocity af 8 km/r&.
Mwing
"London T M a SeMw."
2 hn/wc. 601 q.viptioMi wmcMactiimur and Birkinrhaw.
iion.and air +oubocs h a (thia h
JJ.ES.. D a . 1938.
i@ümate a aii spca-mckeb d
" Tha
Son."
C. G. Abbor
fnxn very high coun(Appkton&.n
3-.
-
v
-
th"g;n'P;I
/ o ~ r n d o,
Ist.r
phnrtay Socuty. Jan.. 1939.
v l0t.R
wh~V~theSnalvelaityoftha
=~,""d%sm""z
rmn (payload plpr s ~ ~ So)
faz v h.beea abont 24.5 km/=
for h u i d he1 rocksb but new deUgpr and hels wüi pumit of conPdarrbly hi ber figura.
(%hydmgen fuCf bu a theoretid uha&
veiocity of 5.2 km/uc m d
moro
mfPl combioltionr are
!-m.p" ~f wa umme v to be 3.3
kmlaa. R wiü bs m to x. How-
.